1.Field of the Invention
This invention relates to transducers which generate and transmit energy in the ultrasonic range, and relates more particularly to transducers using ceramic resonators and operating at third harmonic frequencies.
2.Description of the Relevant Art
Ultrasonic transducers are used for generating and transmitting wave energy of a predetermined frequency for ultrasonic cleaning or other uses. See, for example, U.S. Pat. No. 3,575,383 entitled ULTRASONIC CLEANING SYSTEM, APPARATUS AND METHOD THEREFOR. Transducers of this type can be used, for example, in ultrasonic cleaning equipment. The transducer is typically mounted to the side or the underside of a container which holds liquid, or mounted in a sealed enclosure which is immersed in a liquid in a container made of metal, plastic or glass. One or more transducers are used to energize the liquid with sonic energy. Once energized with the sonic energy, the liquid cavitates.
This type of transducer may be referred to as a xe2x80x9csandwich-typexe2x80x9d transducer because it has a sandwich of a piezoelectric crystal in between two passive masses (head mass and tail mass). This type of transducer is also referred to as a xe2x80x9cstackedxe2x80x9d transducer because it has a stack of component elements. Typically, a head mass (or front driver) is the first component of the stack, the closest to the container or other object to which sonic energy is being transmitted. Then, one or more piezoelectric crystals are stacked onto the head mass, along with one or more electrodes to make electrical contact to the faces of the piezoelectric crystals. Then, a tail mass (or rear driver) is stacked onto the piezoelectric crystal(s), sometimes with electrical insulators to isolate the piezoelectric electrodes to prevent shorting out. These components are typically flat annular disks, with the head mass having a tapped hole. A bolt is inserted through the annulus of the stacked components and is threaded into the tapped hole in the head mass to compress the stack and hold it together.
An alternating current is supplied to the piezoelectric crystal, which expands and contracts. The vibrations of the piezoelectric crystal are transmitted through the head mass to the object being vibrated. Such transducers are used in applications like ultrasonic cleaning, plastic welding, wire bonding, cataract and other medical surgical devices, among others.
The head mass and tail mass are typically made from metals such as stainless steel, aluminum, and titanium. Applicant has proposed using an additional element, called a resonator, between the head mass and the piezoelectric crystal(s) to enhance the output of the transducer relative to conventional transducers, as disclosed in prior patent applications 08/644,843, and 08/792,568. Ceramics, such as alumina (aluminum oxide) and silicon dioxide, are the preferred materials for the resonator.
A piezoelectric crystal has natural frequency that depends upon its size and vibration mode. For example, FIG. 1 illustrates the frequency response of a piezoelectric crystal that has a natural frequency at about 41.5 KHz. The plot shown in FIG. 1 has frequency from 10 KHz to 100 KHz plotted on the x-axis and impedance on a log scale to a maximum of 100 Kxcexa9 plotted on the y-axis. (These scales are also labelled on FIG. 1 at the lower right (xe2x80x9cSTARTxe2x80x9d and xe2x80x9cSTOPxe2x80x9d ) and upper left (xe2x80x9cA MAXxe2x80x9d )). A marker 10 is located at the minimum impedance, indicating a natural frequency of 41.5 KHz and an impedance of 45.2428 xcexa9.
The frequency of an ultrasonic transducer determines the frequency and corresponding size of the ultrasonic waves transmitted to the object. In a cleaning application, for example, a high frequency/short wavelength is needed for cleaning small parts. As the features of parts to be cleaned are reduced in size, higher frequencies/shorter wavelengths are needed so that the ultrasonic waves will be small enough to access those features. If the wavelengths are too big, the features are not reached by the ultrasonic waves. Thus, there is a need for higher frequency ultrasonic transducers for cleaning smaller sized parts.
One way to achieve higher frequencies is to reduce the size of the piezoelectric crystal. Reducing the size, however, has the disadvantage of reducing the amount of ultrasonic energy that can be generated by the piezoelectric crystal. What is needed is a way to increase frequency while maintaining an acceptable level of energy generation.
In accordance with the illustrated preferred embodiment, the present invention provides an improved ultrasonic transducer, and related method, capable of operating efficiently at high frequencies. The ultrasonic transducer of the present invention comprises a head mass, a resonator composed of ceramic material and positioned adjacent to the head mass, a piezoelectric crystal positioned adjacent to the resonator on a side opposite the head mass, a tail mass positioned adjacent to the piezoelectric crystal on a side opposite the resonator, and means for supplying electrical power to the piezoelectric crystal to oscillate the ultrasonic transducer at a third harmonic frequency. The method of using an ultrasonic transducer according to the present invention comprises the steps of providing an ultrasonic transducer having, in order, a head mass, a ceramic resonator, a piezoelectric crystal, and a tail mass in a stacked assembly; and supplying electrical power to the piezoelectric crystal at a frequency to cause the crystal to vibrate the transducer at a third harmonic frequency having low impedance.
The features and advantages described in the specification are not all inclusive, and particularly, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification and claims hereof. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter. For example, the term xe2x80x9charmonic frequencyxe2x80x9d as used herein means a frequency at which the measured impedance of a piezoelectric crystal, either alone or as part of an ultrasonic transducer, has a minimum value. The term xe2x80x9cfirst harmonic frequencyxe2x80x9d or xe2x80x9cnatural frequencyxe2x80x9d is the first frequency at which a significant minimum value of measured impedance first occurs as frequency increases from zero. The term xe2x80x9cthird harmonic frequencyxe2x80x9d is a frequency at which another significant minimum value of measured impedance occurs at or near three times the first harmonic frequency.